Formation of a hard film by a PVD method is advantageous in that the film can be formed at a relatively low temperature. As the pretreatment for forming a hard film on steel parts to be coated with the hard film, composite treatment generally conducted so far comprises previously subjecting the surface of the parts to plasma nitriding to thereby form a nitrided layer on the surfaces, and then forming a hard film on the nitrided layer whereby adhesion and durability of the thus-formed hard film is improved.
According to the existent plasma nitriding, it is possible to uniformly treat the surface of metal parts having relatively simple shapes and having no small unevenness to thereby form a nitrided layer thereon. It is also possible to form a thin hard layer having a thickness of from 5 to 15 .mu.m or so on the surface of metal parts almost without enlarging the surface roughness of the parts.
It is well known to coat the surface of steel materials such as tools and molds with a film of TiN by a PVD method used for ion-plating, thereby improving the wear resistance of the materials. However, metal nitrides such as TiN are defective in that they are easily oxidized at high temperatures and their wear resistance is remarkably lowered owing to the brittle oxide layer formed by oxidation. Therefore, no sufficient effects have been obtained in tools for working hard steel materials or hardly cuttable materials and molds for hot working.
A TiAlN film has been proposed for the improvement of the oxidation problem of TiN. For instance, W. D. Muntz, et al of Reiboltherens Co. reported that TiAlN has good oxidation resistance (refer to J. Vac Sci. Technol., A4, 2717 (1986)).
The above mentioned TiAlN film has improved resistance due to addition of Al, but has a problem in that the mechanical characteristics of the film are worsened as the amount of Al increases. Therefore, when the film is applied to tools, it worsens the quality of them. Since Al is added essentially as a sort of defects in the TiN film, the TiAlN film suffers from lowering of the toughness and has poor mechanical characteristic compared with TiN.
Worsening of the mechanical characteristics caused by the addition of Al becomes conspicuous in a region in which the amount of Al in the TiAlN film exceeds 10 mol% on the basis of metal ingredients. A TiAiN film with an amount of Al of less than 10% exhibits a mechanical characteristic similar with that of a TiN film, but no substantial effect can be obtained for oxidation resistance by the addition of Al at such a low concentration. For compensating the phenomena conflicting to each other, i.e., worsening of the mechanical characteristic and improvement of the oxidation resistance due to the addition of Al, a film of a structure in which an Al concentration changes continuously has been proposed. For instance, Japanese Patent Laid-Open Hei 2-170965 proposes a film structure having gradient Al concentration, in which the Al concentration increases toward the surface and reaches the maximum Al concentration at the uppermost surface.
In the plasma nitriding of the prior art, if metal parts having fine slits, apertures or grooves, or metal parts of different shapes are subjected to plasma nitriding simultaneously, since plasma are localized to specified portions of the metal member, to make the distribution of plasma not uniform, plasma nitriding is not applied uniformly on the surface of the metal parts, or local overheating occurs to abnormally raise the temperature on the surface of the metal parts, thereby deteriorating the characteristic of the parts. Accordingly, it is impossible to apply uniform nitriding to the surface of the metal parts and, even if a hard film is formed by a PVD method on the metal parts that were subjected to not uniform nitriding, adhesion or durability of the film varies greatly.
Further, for improving the adhesion or durability of the film obtained by forming a nitrided layer (diffusion layer) by conducting plasma nitriding, it is necessary to form a nitrided layer of about 30 to 500 .mu.m on the metal parts. However, when a nitrided layer (diffusion layer) of about 30 to 500 .mu.m thickness is formed to the metal parts by the existent plasma nitriding of the prior art, the surface roughness of the metal parts is increased by sputtering, fine powder caused by sputtering or the like is deposited, or a brittle compound layer is formed. Then, when the film is formed over them, adhesion of the film is deteriorated and the adhesion or durability of the hard film formed after the plasma nitriding can not be improved but, rather, the characteristics of the hard film are worsened.